Vehicle fuel systems generally include a provision for controlling evaporative losses from the fuel tank, typically a fuel vapor storage canister that includes a bed of vapor adsorbent material, such as activated charcoal granules. The fuel sitting in the fuel tank, because of temperature differentials, produces a mixture of fuel, air vapor, and even liquid fuel and water components. This mixture, which must be vented in order to control the tank pressure, is routed through the canister, where the fuel vapor component is stored by adsorption in the bed, while the air is vented to atmosphere. Later, engine vacuum is used to purge the canister by drawing in atmospheric air to desorb the fuel vapor and direct it to the engine combustion air intake to be burned.
There are limitations on the efficiency and useful life of the conventional canisters just described. One problem is that there is no particular provision for handling the liquid fuel and water components of the mixture that is fed into the canister from the tank. These liquid components can contaminate the adsorbent bed, reducing its efficiency and useful life. Another shortcoming is that most known canisters both fill and purge the canister from the top. Therefore, with time the adsorbed vapors migrate through the entire bed, with the heaviest concentration of stored vapors sinking toward the bottom. Therefore, when purge occurs, the heaviest concentration of stored vapors is not nearest the purge point, which is not most efficient. In known prior art canisters that do both fill and purge from the bottom of the canister, there are separate fill and purge tubes, so that the heaviest concentration or distribution of stored vapors is still not nearest the purge point.